1. Field of the Invention
The present invention relates generally to the field of electrical machinery and more particularly to a method for identifying phase failure in an asynchronous machine.
2. The Prior Art
The prior art includes German Patent DE 32 28 954 C2, wherein for each phase of an asynchronous motor an electrical signal corresponding to the root-mean-square value of the respective phase current is generated. This signal is compared with a presettable limit value in a comparison circuit. If this limit value is deviated from, a switch-off signal is generated in order to protect the asynchronous motor. Furthermore, a comparison of all the signals which represent the root-mean-square value of all the phase currents is additionally provided in order to generate the switch-off signal in the event of a deviation from a presettable difference. Since, in the case of a power converter-fed asynchronous machine, such a protective device functions only above a specific frequency, in the event of a predetermined frequency limit being undershot, a voltage comparison must additionally be performed in order to check the voltage respectively applied to the asynchronous motor with regard to correspondence to the desired voltage of the converter associated with the instantaneous frequency. The previously known protective device essentially involves generating the switch-off signal in the event of a deviation from the current balance.
A circuit arrangement for identifying a phase failure in a three-phase motor is shown in German patent specification DE 23 03 993 A1. This circuit operates according to a similar method. In this case too the indication of the phase failure functions only above a specific frequency which is determined by the time constant of the filter in the comparison circuit. Therefore, there is no indication of a fault at small rotating field frequencies in the event of a phase failure and, consequently, all of the operating states and/or operating points of an asynchronous machine cannot thereby be monitored.
It is therefore an object of the present invention to provide a method for identifying phase failure in an asynchronous machine which operates over the entire frequency range of the system.
Yet another object of the present invention is to provide a method for identifying phase failure in an asynchronous machine which operates from standstill up to the maximum frequency of the system including direct-current feeding.
The foregoing and other objects and advantages of the present invention will appear more clearly hereinafter.
In accordance with the present invention there is provided a method for identifying the failure of a phase of a converter-controlled three-phase asynchronous machine which includes monitoring of at least two phase currents in a respective monitoring stage of a monitoring circuit. The monitoring circuit which generates a trigger signal in the event of a limit value for the relevant monitoring stage being undershot.
In the first monitoring state in each monitoring stage, the instantaneous value of the phase current is compared with the limit value and the trigger signal is generated during the undershooting of the limit value, after which, given the presence of the trigger signal, a changeover is made from the first to a second monitoring state. In the second monitoring state, the duration of the trigger signal is compared with the angle of rotation swept by the voltage vector of the phase, the voltage vector concomitantly rotating with the rotating field, in which case, in the event of the trigger signal being present over a predetermined rotating field angle, a transition is made to a third monitoring stage. In the third monitoring stage the presence of the trigger signal is checked during a defined delay time interval, phase failure being signaled in the event of said delay time interval being outlasted by the trigger signal.
What is essential for the invention is that the generation of the trigger signal is based on the instantaneous values of the phase currents and the trigger signal is present constantly while the instantaneous value of the phase current falls below the presettable limit value in each of the monitoring stages. For the operating state of an asynchronous machine it is entirely normal for the instantaneous values of the phase currents to fall below the limit value and also pass through a zero value; consequently, the trigger signal is generated each time in these cases in the first monitoring state. This trigger signal, however, has a short time duration. Therefore, in the second monitoring stage, a check is made to see whether the phase current of the relevant phase normally lags behind the voltage vector or desired motor voltage indicator and exceeds the limit value again in a predetermined unit of time, whereby the trigger signal is then obviated. In this normal case, a changeover is immediately made back to the first monitoring state again. However, if the trigger signal is present for longer than the temporal requirement of the voltage vector to sweep the predetermined angle of rotation, then a changeover is made to the third monitoring stage, where the duration of the trigger signal is compared with the defined delay time interval, phase failure being signaled when the duration of the trigger signal exceeds the delay time interval. The filter function in the third monitoring stage prevents premature or incorrect triggering of the phase-failure signal, which is advantageous particularly in the field-weakening range.
For practical operation it is expedient to define the comparison angle of rotation for the voltage vector in the second monitoring state as 90 degrees electrical. The angle of rotation of the voltage vector is equivalent to the rotating field angle, and it is understood that the rotating field angle covered in a unit of time is proportional to the rotating field frequency. In normal operation of the asynchronous machine, the phase current changes during the sweeping of the predetermined angle of rotation by the voltage vector and exceeds the present limit value. If, by contrast, the phase current does not change during this period of time, the asynchronous machine is deviating from its normal operating behavior and a fault is probably present, which fault results in the signaling of the phase failure and switching off of the asynchronous machine in any case when the fault criterion is fulfilled in the third monitoring stage.
The method according to the present invention is particularly suitable for converter control of the entire drive system, whereby the corresponding values can be picked off at the converter output. It is not necessary to monitor all three phases in the case of the feeding of a three-phase asynchronous machine, since the instantaneous value of the third phase current can in each case be calculated from two detected phase currents. Thus, a further conceivable case is one in which during the monitoring of two of the three phase currents, both instantaneous values of the monitored phase currents are smaller than the comparator threshold, that is to say lie below the predetermined limit value. In this situation, a separate, independent time measurement is started from zero and outlasts a separate delay time interval. If the state remains constant, in other words if the instantaneous values of the phase currents do not exceed the comparator threshold, phase failure of the asynchronous machine is likewise signaled and, if appropriate, shutdown is initiated. However, this can also means that no asynchronous machine at all is connected to the converter or the connection of the asynchronous machine is not in order.
Overall, the method according to the invention is suitable both for current-regulated and for voltage-controlled converters, and the entire moment range of the connected asynchronous machine is covered, including no-load running, in any desired direction of rotation; in other words, the monitoring functions during operation in all four quadrants of the M/n characteristic curve of the asynchronous machine. Moreover, a simultaneous failure of a plurality of phases of the asynchronous machine is identified and, ultimately, the method according to the invention can also be applied analogously to other polyphase systems.